The magnitude of neurobehavioral performance failure during sleep deprivation varies widely among individuals, and there are marked inter-individual differences in the structure of subsequent recovery sleep as well. Using a repeated total sleep deprivation (TSD) paradigm, the proposed project seeks to quantify the trait component of inter-individual differences in the response to sleep loss. In order to examine the malleability of the trait component, the magnitude of the trait component will also be compared with the magnitude of the response to a change in workload state. In addition, potential predictors of individual responses to sleep loss will be identified. A total of n=39 subjects will be exposed, on three consecutive occasions, to 36 hours of controlled TSD in a laboratory. Each 36-hour TSD will be followed by two 12-hour recovery sleep opportunities. Throughout the experiment, waking neurobehavioral performance will be monitored every two hours at fixed times by means of standardized behavioral tests, while baseline and recovery sleep will be studied with polysomnography (PSG) and electroencephalogram (EEG) power-spectral analysis. For each subject, two of the three TSD periods will involve moderate workload, while the other TSD period will have high (i.e. double-duration) workload, in randomized counterbalanced order. The investigation has five specific aims: (1) Quantify the magnitude and importance of inter-individual variability in neurobehavioral performance failure during TSD and the structure of subsequent recovery sleep; (2) Quantify the extent to which neurobehavioral performance failure during TSD involves a trait response and/or a state response to workload; (3) Quantify the extent to which the structure of recovery sleep following TSD involves a trait response and/or a state response to prior workload; (4) Explore the relationship between individuals' magnitude of neurobehavioral performance failure during TSD and the structure of their subsequent recovery sleep; and (5) Identify predictors of individuals' magnitude of neurobehavioral performance failure during TSD and the structure of their subsequent recovery sleep, by examining six predictor domains: baseline neurobehavioral performance, baseline sleep architecture, circadian rhythmicity, physical characteristics, psychological parameters, and demographic statistics. The results of this investigation will fill a critical gap in our understanding of inter-individual differences in the response to sleep loss, and the factors that determine these differences. The research will be relevant for the understanding, remediation and prevention of the potentially serious effects of sleep deprivation in the millions of individuals who are occasionally or routinely exposed to sleep loss due to medical condition, occupation or lifestyle.